This invention relates to a piezoelectric actuator using, as a driving source, a piezoelectric element to be deformed by the supply of a driving signal, a liquid jetting head comprising such a piezoelectric actuator, a method of manufacturing the piezoelectric element.
A piezoelectric actuator is a member for using a piezoelectric element as a driving source and flexing a vibration plate to change the volume of a pressure chamber by the deformation of the piezoelectric element, and is suitably utilized as a liquid jetting head or a member of a micropump, for example. Here, the piezoelectric element comprised of a piezoelectric material which shows a piezoelectric effect, such as a piezoelectric ceramics made by compressing and baking metal oxide powder (e.g., BaTiO3, PbZrO3, PbTiO3), a piezoelectric polymer film using polymer compound. The liquid jetting head serves to eject a liquid droplet from a nozzle orifice, and includes a recording head to be used in an image recording apparatus such as a printer, a liquid crystal jetting head to be used for manufacturing a liquid crystal display, and a coloring material jetting head to be used for manufacturing a color filter, for example. Moreover, the micropump is a microminiaturized pump capable of treating a very small amount of liquid.
The vibration plate is a very thin member having a thickness of approximately 5 to 6 μm, and the piezoelectric element has a single layer structure which has been used practically, and has a thickness of approximately 15 μm. Accordingly, the total thickness of the vibration plate and the piezoelectric element is approximately 20 μm. For this reason, the compliance of a deformed portion in a pressure chamber, that is, the compliance of the vibration plate and the piezoelectric element becomes large. Consequently, there is a possibility that a volume might be changed with a slight variation in liquid pressure present in the pressure chamber. Moreover, if the compliance of the deformed portion is large, the responsibility of the piezoelectric element (the responsibility of the deformation) for a supplied driving signal is deteriorated so that high frequency driving is hard to perform.
It can be supposed that the thickness of the piezoelectric element is set to be larger in order to reduce the compliance of the deformed portion. If the thickness is simply increased, a driving voltage is to be raised correspondingly and is not suitable for the high frequency driving. Moreover, the deformed portion is hard to flex. Consequently, the amount of a pressure fluctuation of the liquid becomes insufficient.
Therefore, it is supposed that a piezoelectric element having a multilayer structure disclosed in Japanese Patent Publication No. 2-289352A and Japanese Patent Publication No. 10-34924A is used. In the piezoelectric element, a piezoelectric layer has a two-layer structure having an upper piezoelectric layer and a lower piezoelectric layer and a drive electrode is formed on a boundary between the upper piezoelectric layer and the lower piezoelectric layer, and furthermore, an upper common electrode is formed on the outer surface of the upper piezoelectric layer and a lower common electrode is formed on the outer surface of the lower piezoelectric layer.
The piezoelectric element has the drive electrode provided on the boundary between the upper piezoelectric layer and the lower piezoelectric layer. For this reason, an electric field having an intensity determined by a distance from the drive electrode to each common electrode (the thickness of each piezoelectric layer) and a potential difference between the drive electrode and each common electrode is given to the piezoelectric element of each layer. As compared with a conventional piezoelectric element having a single layer structure including a single piezoelectric layer interposed between the common electrode and the drive electrode, that is, a piezoelectric element used practically, therefore, the piezoelectric element can be deformed largely at the same driving voltage as that in the conventional element even if the thickness of the whole piezoelectric element is slightly increased to enhance a rigidity.
However, it is hard to obtain desirable characteristics by simply using the piezoelectric element having the multilayer structure described in each publication. For example, the ejecting characteristic or feeding characteristic of liquid is varied or a balance between the high frequency driving of the piezoelectric element and the ejecting amount (feeding amount) of the liquid cannot be taken. Moreover, there is also a problem in that a disconnection is apt to be caused on an electrode. For this reason, a piezoelectric element having a single layer structure including a single piezoelectric layer interposed between a common electrode and a drive electrode is inevitably used as an actual product.